Small-bore hollow waveguides for delivery of 3-mm laser radiation

Small-bore hollow waveguides for delivery of 3-mm laser radiation Rebecca L. Kozodoy, Antonio T. Pagkalinawan, and James A. Harrington Flexible hollow glass waveguides with bore diameters as small as 250 µm have been developed for 3-µm laser delivery. All the guides exhibit straight losses between 0.10 and 1.73 db@m, and the loss increases to between 2.4 and 5.1 db@m upon bending 1moftheguides into 15-cm-diameter coils. This behavior is shown to depend strongly on the launch conditions and mode quality of the input beam. The waveguides are capable of efficiently delivering up to 8WofEr:YAG laser power with proper input coupling, and they are suitable for use in both medical and industrial applications. Key words: Erbium YAG, erbium-ysgg, fiber optics, hollow waveguides, infrared waveguides, power delivery. r 1996 Optical Society of America 1. Introduction Near-infrared lasers such as Er:YAG and Er:YSGG that operate at waveguides near 3 µm are gaining in popularity for various applications. Because this wavelength range matches the strongest absorption band of water in tissue, these lasers may be used for precise cutting and ablation of tissue in surgery. Industrial uses of these lasers are also being explored. As a result, much research in recent years has focused on developing a commercial fiber-optic delivery system for 3-µm lasers that meets the stringent requirements of optical efficiency, durability, and easy handling. To this end, solid-core IR transmitting fibers have been produced from both glass and crystalline materials, including heavy-metal fluorides, low-molecular-weight chalcogenides 1As 2 S 3 2, silver halides, and sapphire. 1 7 Most of these have significant drawbacks, however, such as low laserdamage thresholds, poor chemical and mechanical properties, or difficult and expensive fabrication. Reasonable success has been achieved with dielectriccoated metallic hollow waveguides, 2,8 13 but the large bore diameters of these devices 1.1 mm2limit their flexibility and result in propagation of higher-order modes. Recently we developed a new type of highly flexible small-bore hollow waveguide 1bore sizes from 250 to The authors are with the Fiber Optics Research Program, Rutgers University, Piscataway, New Jersey 08855. Received 7 August 1995. 0003-6935@96@071077-06$06.00@0 r 1996 Optical Society of America 1000 µm2, consisting of a metal and a dielectric coating deposited inside silica tubing, for delivery of 3-µm laser radiation. 8 10 This study characterizes the transmission properties of this waveguide in both straight and bent configurations for Er:YAG 12.94-µm2 and Er:YSGG 12.79-µm2 laser light, and it discusses its power handling capabilities. Additionally, because most Er:YAG and Er:YSGG lasers generate highly multimode beams, this study also investigates the effect of input-beam mode quality on the output distribution and attenuation behavior of this new waveguide. 2. Fabrication The waveguides used in this study were produced by liquid phase deposition of silver on the internal surface of small-bore silica tubing, followed by reactive formation of a silver iodide layer. By adjustment of the thickness of the dielectric AgI layer, the waveguides were tailored for lowest loss in the 3-µm wavelength region. The fabrication process is relatively simple and inexpensive, and the yield is currently greater than 90%. A detailed description of this fabrication process can be found elsewhere. 9,10 For this study, hollow waveguides were fabricated with bore diameters of 250, 320, 530, 700, and 1000 µm, in lengths between 1 and 2 m. 3. Optical Characterization A. Infrared Attenuation Spectra Infrared attenuation spectra for each waveguide were obtained with a Fourier transform IR spectrometer 1Perkin-Elmer Model 1725X2. In all measure- 1 March 1996 @ Vol. 35, No. 7 @ APPLIED OPTICS 1077

ments the incoherent light was coupled into the waveguides by the use of an off-axis parabolic mirror of focal length,2.5 cm. Figure 1 shows the spectral response of a typical 700-µm-bore hollow waveguide designed for minimum loss near 3 µm. The IR loss spectra of the other waveguides used in this study are similar in shape. The small peak at 4.25 µm and the structure between 5 and 7 µm result from absorption of the light by CO 2 and water, respectively, in the air within the waveguide; in measurements in which the waveguide bore is flushed with nitrogen, a fairly flat response beyond 3 µm is normally seen. This suggests that these hollow waveguides would be useful for broadband IR applications. The attenuation measured with a spectrometer greatly exceeds that measured with a laser, as discussed below. B. Straight Losses Both an Er:YAG and an Er:YSGG laser were used to measure the attenuation of the hollow waveguides and to examine the influence of the input launch conditions on the loss behavior. Although both lasers produced multimode outputs, their beam diameters and mode qualities varied as shown in Fig. 2. This figure shows the spatial profile of the beams measured with a 32 3 32 matrix pyroelectric detector array 1element size: 0.8 mm 3 0.8 mm2. The highly apertured Er:YAG laser yielded a relatively low-order modal distribution and small beam-output diameter as depicted in Fig. 21a2. Figures 21b2 and 21c2 show the output of an Er:YSGG limited by a 2-mm aperture and a 3.75-mm aperture placed within the laser cavity. Both apertures produce Er:YSGG output beams larger than the Er:YAG output beam. The modal distribution of the Er:YSGG is also poorer for both apertures, with the 3.75-mm aperture producing the most multimode beam, as seen in Fig. 21c2. The coupling of these input beams into the waveguides required the use of different lenses for maximum transmission. In general, longer focal length lenses excite lower-order modes within the wave- 1a2 1b2 1c2 Fig. 1. Spectral loss of a typical hollow waveguide designed for use near 3 µm. Fig. 2. Beam profile of laser outputs: 1a2 Er:YAG, 1b2 Er:YSGG with 2-mm aperture, 1c2 Er:YSGG with 3.75-mm aperture. 1078 APPLIED OPTICS @ Vol. 35, No. 7 @ 1 March 1996

guides, decreasing the interaction of the light with the waveguide walls and minimizing the attenuation. However, shorter focal length lenses produce smaller beam diameters. Because minimal loss is achieved when the ratio of the spot size to the bore size is,0.6, we always attempted to optimize the coupling by using appropriate focal length lenses to approximate this ratio. 14,15 As the laser mode quality became poorer and the beam diameter increased, progressively shorter focal length lenses were required to focus the input beam to a spot size smaller than the waveguide bore. In fact, we were unable to focus the light from the 3.75-mm-aperture Er:YSGG to a small enough spot to couple into the 250-µmbore waveguide with our shortest focal length lens 3 f 5 0.5 in. 1,1.27 cm24. Figure 3 shows the attenuation of five hollow waveguides measured as a function of bore diameter, using the lasers and launch conditions described above. Also included in Fig. 3 for comparison are the theoretical losses for these waveguides calculated at the Er:YAG wavelength of 2.94 µm, assuming that only the HE 11 mode propagates. Losses calculated for Er:YSGG light at 2.79 µm are slightly lower than these and are not shown. These calculations were performed by the use of an equation derived by Miyagi and Kawakami, 16 which gives the attenuation coefficient, a, for straight waveguides as a 5 1 U nm 2p 2 2 l 2 a 3 1 n n 2 1 k 22 1 2 3 1 1 n 1 2 1n 12 2 12 1@242. 112 Fig. 3. Transmission loss of straight hollow waveguides as a function of bore diameter for laser inputs shown in Fig. 2. The theoretical loss assuming HE 11 mode propagation of 2.94-µm light is included for comparison. Here, U nm is a modal parameter equal to 2.405 for the lowest-order HE 11 mode, l is the wavelength, a is the bore radius, n and k are the optical constants of the metal film, and n 1 is the refractive index for the dielectric film 1at,3 µm the extinction coefficient of the dielectric is approximately zero2. Equation 112 thus predicts that the attenuation of the straight hollow waveguide should vary as 1@a 3 for singlemode propagation, and that higher-order modes are strongly attenuated as U nm increases. Figure 3 illustrates several significant results. First, the measured attenuation increasingly exceeds the theoretical loss as the quality of the beam launched into the waveguide degrades. In addition, the measured attenuation increases as the bore diameter is reduced, but the increase in loss does not follow a 1@a 3 dependence. As the f-number of the launched beam and its mode quality improves, though, the increase in attenuation with bore diameter more closely approximates this 1@a 3 behavior. Figure 3 also demonstrates that for reasonably good input mode and launch conditions, the attenuation of these hollow waveguides may be less than 2 db@m, even for bore diameters as small as 250 µm. The lack of correlation between the theoretical and the experimental attenuation in Fig. 3, as well as the high loss measured with the Fourier transform IR spectrometer in Fig. 1, is a direct result of the multimode input beam and the less than ideal coupling of the light into the waveguides. From Eq. 112, we see that as higher-order modes are excited within the guides, the attenuation increases as the square of the modal parameter. Previous research with hollow dielectric- and metallic-coated waveguides and CO 2 lasers with near-gaussian beams has demonstrated a much better agreement between theory and experiment and provides additional evidence that if the modal quality of the input beam and the launch conditions were optimized, better transmission efficiency would be obtained. 9,17 Furthermore, it is likely that a 1@a 3 dependence is not seen for the poorer quality beams because the modal distribution within the waveguide is changing over the length as higher-order modes are damped out. In real applications, the launch conditions are critical in maximizing the efficiency of the waveguide transmission. C. Bending Losses Previous theoretical and experimental research has demonstrated that hollow waveguides exhibit an additional loss caused by bending that varies inversely with the bend radius R. 8 11,18 As a way to characterize the optical behavior of the new hollow waveguides further, the increase in attenuation on bending was evaluated. For these measurements the input ends of the waveguides were held straight, and approximately 1moftheguide was bent to a uniform bending radius. Figure 4 compares the bending loss of a 530-µm-bore waveguide for the Er:YSGG with the 2- and 3.75-mm apertures. Each data set shows a strong linear dependence on curvature, as predicted theoretically, with linear regression R 2 values greater than 0.98. Furthermore, the two lines are displaced from one another in the y direction, but they have similar slopes. As in Fig. 3, the input beam with the smaller f-number launch 13.75-mm aperture2 is more highly attenuated than the input beam with the larger f-number launch. 1 March 1996 @ Vol. 35, No. 7 @ APPLIED OPTICS 1079

Fig. 4. Bending loss of 530-µm-bore waveguide for Er:YSGG with 2-mm and with 3.75-mm apertures. However, the similarity in slope indicates that the additional loss caused by bending is comparable for both launch conditions. Figure 5 illustrates the bending loss as a function of curvature 11@R2 for each of the different bore sizes as measured with the Er:YSGG with the 2-mm aperture. The losses shown here are quite low, generally less than 5.2 db@m, even when the waveguides were bent into 15-cm-diameter coils. These data also show that as the waveguide bore size is reduced, the attenuation of the straight waveguides increases but the slope of the bending loss curves decreases. In other words, less additional loss is incurred on bending the smaller-bore waveguides than the larger-bore ones. The decrease in additional loss caused by bending as the bore size decreases can be attributed to higher-order mode filtering. In general, loss results from the interaction of the light with the waveguide wall, so that from a ray optics viewpoint, higherorder modes generate higher loss because of an increased number of bounces of the light within the guide. Similarly, as the waveguide is bent, the angle of incidence of the light with the waveguide wall is increased, leading to a greater interaction of Fig. 5. Measured bending loss of 250-, 320-, 530-, 700-, and 1000-µm-bore waveguides for Er:YSGG with 2-mm aperture. the light with the waveguide and a greater loss. Mode filtering occurs for the smaller-bore guides because with highly multimode input beams, lower f-number lenses must be used to produce small incident spot sizes. These lenses produce a greater beam divergence and excite higher-order modes within the waveguide, increasing the total attenuation. Equation 112 reflects this increased loss with a higher mode because a varies as 1U nm 2 2 @a 3. The smaller-bore guides are therefore able to filter out the higher-order modes more effectively within a short length. As can be seen in Fig. 5, the losses for the larger and smaller bores can eventually cross and become equal, once only the lowest order modes remain. D. Output-Beam Profile Many surgical and industrial applications require good spatial quality of the output laser beams to maintain a small spot size for well-controlled cutting and ablation. The modal output of the new hollow waveguides was examined with the same 32 3 32 matrix pyroelectric detector array used above to measure the laser outputs. Figures 61a2 and 61b2 show the output-beam profiles of a 1000- and 320-µmbore waveguides, using the Er:YSGG laser with the 3.75-mm aperture to provide the input beam shown in Fig. 21c2. Although the input beam is of poor spatial quality with many higher-order modes, the 320-µm-bore waveguide produces a much cleaner output beam than the output beam from the 1000-µmbore waveguide. The small-bore waveguides thus filter out many of the higher-order modes, providing further evidence for the decrease in additional loss on bending with decreasing bore diameter in Fig. 5. E. High-Power Handling Ability Lasers in the 3-µm wavelength region typically operate at output powers of less than 10 W. Many of the solid-core fibers that have been developed for 3-µm laser delivery are limited in their ability to transmit light at these power levels because they have low laser-damage thresholds. 1 3,6,7 Hollow waveguides have much higher laser-damage thresholds because the light is primarily confined to an air core rather than solid material, and larger-bore hollow dielectric-coated metallic guides have been shown to deliver relatively high average powers of Er:YAG light. 12 We studied the ability of the smallbore hollow waveguides to handle high-power transmission, using a high-power, highly multimode Er:YAG laser. Figure 7 shows the results of these measurements for 1000-, 700-, and 530-µm-bore waveguides. The 1000-µm-bore waveguides were consistently able to deliver nearly 8 W for maximum input powers less than 10 W. However, the smallerbore guides were limited to less than 3 W output because we were unable to focus the multimode beam to a small enough diameter to couple into the guides with our shortest focal length 3 f 5 0.5 in. 1,1.27 cm24 lens without damaging the input end. 1080 APPLIED OPTICS @ Vol. 35, No. 7 @ 1 March 1996

As discussed above, these improvements in the input conditions would excite lower-order modes within the waveguides, minimizing attenuation and damage. This is corroborated by the fact that similar hollow waveguides 1with slightly thicker dielectric layers to provide minimal loss at 10.6 µm2 have been able to deliver over 35 W of near-gaussian CO 2 radiation without active cooling, and greater than 1.01 kw of multimode CO 2 light in a water-cooled configuration. 9 1a2 1b2 Fig. 6. Beam profiles of 1a2 1000-µm and 1b2 320-µm-bore straight waveguides for Er:YSGG with a 2-mm aperture. It is likely that the new hollow waveguides would be capable of transmitting even higher average power levels of 3-µm radiation than shown here, if a better quality laser beam and launch were used. Fig. 7. Er:YAG laser power delivery for 1000-, 700-, and 530-µmbore waveguides. 4. Conclusion Hollow metallic- and dielectric-coated glass waveguides with bore diameters as small as 250 µm have been developed for delivery of 3-µm radiation. These guides have been shown to provide efficient transmission not only in this spectral region, but also over a broad wavelength range extending into the mid-ir. All of the guides exhibit straight losses between 0.10 and 1.73 db@m, and the loss increases to between 2.4 and 5.1 db@m upon bending1mofthe highly flexible guides into 15-cm-diameter coils. The smaller-bore guides were seen to filter out the highest order modes, resulting in less additional bending loss than the larger-bore guides. It is important to note that the optical properties of these hollow waveguides were shown to depend strongly on the launch conditions and mode quality of the input beam. With proper input coupling, however, the waveguides are capable of efficiently delivering upto8wof2.94-µm light and are suitable for use in both medical and industrial applications. The authors are indebted to K. Matsuura for fabricating the hollow waveguides used in this study, to C. Rabii and J. C. Jensen for their assistance with the experimental research, and to Y. Matsuura for his help with the theoretical calculations. References 1. G. N. Merberg, Current status of infrared fiber optics for medical laser power delivery, Lasers Surg. Med. 13, 572 576 119932. 2. U. Kubo, Y. Hashishin, H. Tanaka, and T. Mochizuki, Development of optical fiber for medical Er:YAG laser, in Optical Fibers in Medicine VII, A. Katzir, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1649, 34 40 119922. 3. G. Merberg, M. Shahriari, J. A. Harrington, and G. H. Sigel, Jr., Evaluation of crystalline and chemically durable glass fibers for Er:YAG laser delivery systems, in Infrared Fiber Optics II, J. A. Harrington and A. Katzir, eds., Proc. Soc. Photo-Opt. Instrum. Eng. 1228, 216 233 119902. 4. A. R. Hilton, Chalcogenide glass optical fibers, in Infrared Fiber Optics III, J. A. Harrington and A. Katzir, eds., Proc. Soc. Photo-Opt. Instrum. Eng. 1591, 34 42 119912. 5. A. Katzir and R. Arieli, Long wavelength infrared optical fibers, J. Non-Cryst. Solids 47, 149 158 119822. 6. G. N. Merberg and J. A. Harrington, Optical and mechanical properties of single-crystal sapphire fibers, Appl. Opt. 32, 3201 3209 119932. 7. D. H. Jundt, M. M. Fejer, and R. L. Byer, Characterization of single-crystal sapphire fibers for optical power delivery systems, Appl. Phys. Lett. 55, 2170 2172 119892. 8. J. A. Harrington and Y. Matsuura, Review of hollow wave- 1 March 1996 @ Vol. 35, No. 7 @ APPLIED OPTICS 1081

guide technology, in Biomedical Optoelectronic Instrumentation, A. Katzir, J. A. Harrington, and D. M. Harris, eds., Proc. Soc. Photo-Opt. Instrum. Eng. 2396, 4 14 119952. 9. T. Abel, J. Hirsch, and J. A. Harrington, Hollow glass waveguides for broadband infrared transmission, Opt. Lett. 19, 1034 1036 119942. 10. Y. Matsuura, T. Abel, and J. A. Harrington, Optical properties of small-bore hollow glass waveguides, Appl. Opt. 34, 6842 6847 119952. 11. Y. Matsuura, A. Hongo, and M. Miyagi, Dielectric-coated metallic hollow waveguide for 3-µm Er:YAG, 5-µm CO, and 10.6-µm CO 2 laser light transmission, Appl. Opt. 29, 2213 2214 119902. 12. Y. Matsuura and M. Miyagi, Er:YAG, CO, and CO 2 laser delivery by ZnS-coated Ag hollow waveguides, Appl. Opt. 32, 6598 6601 119932. 13. N. Croitoru, J. Dror, and I. Gannot, Characterization of hollow fibers for the transmission of infrared radiation, Appl. Opt. 29, 1805 1809 119902. 14. C. C. Gregory and J. A. Harrington, Attenuation, modal, and polarization properties of n, 1, hollow dielectric waveguides, Appl. Opt. 32, 5302 5309 119932. 15. A. Hongo, M. Miyagi, K. Sakamoto, S. Karasawa, and S. Nishida, Excitation dependent losses and temperature increase in various hollow waveguides at 10.6 µm, Opt. Laser Technol. 19, 214 216 119872. 16. M. Miyagi and S. Kawakami, Design theory of dielectriccoated circular metallic waveguides for infrared transmission, J. Lightwave Technol. LT-2, 116 126 119842. 17. S. J. Saggese, J. A. Harrington, and G. H. Sigel, Jr., Attenuation of incoherent infrared radiation in hollow sapphire and silica waveguides, Opt. Lett. 16, 27 29 119912. 18. M. Miyagi and S. Karasawa, Waveguide losses in sharply bent circular hollow waveguides, Appl. Opt. 29, 367 370 119902. 1082 APPLIED OPTICS @ Vol. 35, No. 7 @ 1 March 1996